LTE (Long Term Evolution) is a 4G wireless broadband technology. It is technological successor of 2G technologies like GSM and 3G technologies like UMTS.
In current LTE specification, RRC (Radio Resource Control) in an eNB (Evolved Node B) controls security parameters that are used in generation of security or ciphering key streams in control plane and user plane. In user plane there are security requirements, which set that repetition of packet ciphering key streams is not allowed.
Following input parameters are used for generating the ciphering key streams: COUNT, BEARER, DIRECTION, LENGTH and KEY. The BEARER and COUNT parameters are used for avoiding repetition of the ciphering key stream. A unique BEARER value is assigned for each new user plane bearer that is to be established and value of the COUNT parameter is then incremented for each ciphered block to ensure unique combination of input parameters.
The value space of the BEARER and COUNT parameters is however limited. When all available BEARER values have been consumed (the number of established bearers exceeds the number of available BEARER values) or value of the COUNT parameter reaches its maximum (the wrap-around point) for a given bearer, there is a need to change the value of some other input parameter to ensure unique combination of input parameters (i.e. to avoid repetition of the ciphering key stream). The current solution to this is to perform an intra-cell handover, which renews the KEY parameter, and thereby opens the value spaces of the BEARER and COUNT parameters for reuse. Present solutions of using the BEARER and COUNT parameters to prevent key stream repetition are discussed in more detail e.g. in 3GPP TSG-RAN WG2 meeting contribution 3GPP R2-086174, November 2008.
A downside of using intra-cell handovers is that regular handovers and intra-cell handovers are complex operations and require additional signalling in eNB and UE (User Equipment). Additionally, handovers may cause some extra jitter/delay or packet loss for user plane bearers. Thus there is room for further considerations.